The presence of echoes, or backward propagating waves, in piezoelectric crystals has recently been reported, see Shiren et al "Echo Phenomenon in Piezoelectric Crystals," Physical Review Letters, Vol. 31, No. 13, Sept. 24, 1973 (pages 819-822) and Melcher et al "Parametric Field Echoes in Cds" (pages 558-560). Melcher et al have also suggested that this effect can be applied to an optical scanner for converting optical information to amplitude modulated radio frequency information suitable for data transmission, see the co-pending Melcher and Shiren application Ser. No. 643,971, filed Dec. 29, 1975. Such an apparatus could be constructed, according to the Melcher et al suggestion, by employing a piezoelectric substrate upon which a photosensitive semiconductor is depositioned, the combination of which structure is included between a pair of electrodes, one of which is transparent. An optical image is focused on the photosensitive semiconductor and an acoustic wave is propagated in the piezoelectric substrate. When the acoustic wave reaches the location of the photosensitive semiconductor an electric field is applied thereto exciting charge carriers which are trapped at impurity states in the photosensitive semiconductor in a spatial distribution reflecting the light intensity distribution. The electric field applied to the photosensitive semiconductor redistributes the space charge into a cosine gratng which exists only in the areas of the photosensitive semiconductor which were illuminated. At a later time, a second electric field pulse is applied to excite the grating generating forward and backward surface acoustic waves, either or both of which can be converted by transducers into RF signals which are amplitude modulated with optical information. Melcher et al further suggested that a backward acoustic wave could be generated by a single electric field pulse. The magnitude of this backward wave is determined by the number density of filled impurity states (traps) in the crystals. The local number density is in turn determined by the light intensity at a given point in the crystal. Therefore, the backward wave in such a device is amplitude modulated by the light intensity.
Although such devices appear to be feasible they do present a number of disadvantages. In the first place, since the modified device as suggested by Melcher et al, relies upon charge carriers excited by the light intensity in a photosensitive semiconductor, the same effect results in rather high losses to the acoustic surface wave pulse. In addition, the necessity for the material chosen as the photosensitive semiconductor to exhibit both photosensitive and semiconducting properties makes it difficult to select a material having optimal characteristics as both a photoconductor and a semiconductor.
It is therefore one object of the present invention to provide an optical scanner relying upon echoes in piezoelectric crystals which does not rely upon excitation of charge carriers in a photosensitive semiconductor, and which thus does not result in attenuation of the acoustic wave for this reason. It is another object of the present invention to provide an optical scanner of the foregoing type which does not rely upon excitation of charge carriers in a photosensitive semiconductor so as to allow optimal selection of materials for the device.